[PM] Urgent discarding policy - only discard on critical pressure

Bug: 1030399
Change-Id: I98304a72ef053569988d82efacfb564f25d4717c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007981
Auto-Submit: Sébastien Marchand <sebmarchand@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733418}

chrome/browser/performance_manager/graph/policies/urgent_page_discarding_policy.cc

@@ -9,6 +9,7 @@
 #include "base/containers/flat_map.h"
 #include "base/containers/flat_set.h"
 #include "base/feature_list.h"
+#include "base/memory/memory_pressure_listener.h"
 #include "base/sequence_checker.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
@@ -89,6 +90,11 @@ void UrgentPageDiscardingPolicy::OnMemoryPressure(
     base::MemoryPressureListener::MemoryPressureLevel level) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
 
+  if (level != base::MemoryPressureListener::MemoryPressureLevel::
+                   MEMORY_PRESSURE_LEVEL_CRITICAL) {
+    return;
+  }
+
   // The Memory Pressure Monitor will send notifications at regular interval,
   // it's important to unregister the pressure listener to ensure that we don't
   // reply to multiple notifications at the same time.

chrome/browser/performance_manager/graph/policies/urgent_page_discarding_policy_unittest.cc

@@ -159,6 +159,9 @@ class UrgentPageDiscardingPolicyTest : public GraphTestHarness {
   FrameNodeImpl* frame_node() { return main_frame_node_.get(); }
   void ResetFrameNode() { main_frame_node_.reset(); }
   MockPageDiscarder* discarder() { return mock_discarder_; }
+  util::test::FakeMemoryPressureMonitor* mem_pressure_monitor() {
+    return &mem_pressure_monitor_;
+  }
 
  protected:
   // Make sure that |page_node| is discardable.
@@ -413,5 +416,14 @@ TEST_F(UrgentPageDiscardingPolicyTest,
   testing::Mock::VerifyAndClearExpectations(discarder());
 }
 
+TEST_F(UrgentPageDiscardingPolicyTest, NoDiscardOnModeratePressure) {
+  // No tab should be discarded on moderate pressure.
+  mem_pressure_monitor()->SetAndNotifyMemoryPressure(
+      base::MemoryPressureListener::MemoryPressureLevel::
+          MEMORY_PRESSURE_LEVEL_MODERATE);
+  task_env().RunUntilIdle();
+  testing::Mock::VerifyAndClearExpectations(discarder());
+}
+
 }  // namespace policies
 }  // namespace performance_manager